1. Field of the Invention
The present invention relates to a positive active material composition for a lithium-sulfur battery and a lithium-sulfur battery fabricated using the same and, more specifically, to a positive active material composition for a lithium-sulfur battery with good adhesion to a current collector and which can provide a lithium-sulfur battery exhibiting good cycle life characteristics.
2. Description of the Related Art
As technologies in the field of electronics have been tremendously improved, the use of various portable electronic devices including notebook computers and mobile communication devices has become widespread in recent years. Especially, as portable electronics gets smaller, lighter and thinner, there is a growing need for batteries with a high energy density. There is also a growing need for inexpensive, safe and environmentally friendly batteries.
Lithium-sulfur batteries are attractive candidates for satisfying these requirements because sulfur is cheaper, environmentally friendly and exhibits good energy density, where lithium has a theoretical electrical capacity per weight of 3830 mAh/g and sulfur has a theoretical electrical capacity per unit weight of 1675 mAh/g.
A lithium-sulfur secondary battery uses sulfur-based compounds with a sulfur-sulfur bond as positive active material, and a lithium metal or carbon based compounds as a negative active material. The carbon-based compounds in which intercalation chemistry occurs, include graphite, graphite intercalation compounds, carbonaceous materials, and carbonaceous materials inserted with lithium. Upon discharge (electrochemical reduction), a sulfur-sulfur bond break down occurs to result in a decrease in the oxidation number of S, and upon recharging (electrochemical oxidation), a sulfur-sulfur bond formation occurs to lead to an increase in the oxidation number of S.
A lithium-sulfur battery has a shortcoming in that discharge products (lithium polysulfide, etc.) of positive active materials are dissolved in electrolyte solvents and diffused away from a positive electrode. This renders poor cell performances such as low capacity and low cycle life characteristics.
One approach to solve the dissolving problem is a solid-state or gel-state positive electrode proposed by Chu et al. (U.S. Pat. No. 5,523,179 and related continuation in-part applications), or a liquid electrode called a “catholyte” (U.S. Pat. No. 6,030,720).
Another approach is that sulfur active materials are obtained from covering a electroactive sulfur-containing material with an electroactive transition metal chalcogenide (U.S. Pat. No. 5,919,587, Skotheim et al). In this method, an electroactive sulfur-containing material is added to a solution of electroactive transition metal chalcogenide followed by the addition of a conductive agent to prepare a positive active material composition. The positive active material composition is then coated on a current collector. The transition metal chalcogenide effectively encapsulates or embeds the electroactive sulfur-containing material and chemically and electrostatically embeds the electroactive sulfur-containing material, thereby preventing the electroactive sulfur-containing material from leaving the current collector. However, the transition metal chalcogenide does not effectively retard the electroactive sulfur-containing material from leaving the current collector. U.S. Pat. No. 5,961,672 (Moltech) teaches a stabilizing film deposited-lithium anode to improve cycle life characteristics and safety. This patent does not disclose the effect of the binder TEFLONTM (PTFE-K30, Dupont) on the battery performances.
However, there has been little investigation on a binder, used in the composition of the positive electrode, which is very critical to the utilization of sulfur and the cycle life of a lithium sulfur battery.